In general, an optical system for compensating or correcting vision can make use of two main types of lens.
An aperture lens is designed to exercise the optical function that gives it its correcting or compensating power either in or against the eye. It is typically an intraocular lens or a corneal contact lens. The optical function exerted on the eye by such an aperture lens depends on the aperture of the pupil, but is independent of the direction in which the eye is looking.
In contrast, a field lens is designed to perform its optical function at a distance from the eye. It is typically constituted by a lens in a pair of spectacles mounted in a frame placed on the nose of the wearer. It may also be constituted by a monocle. The optical function performed on the eye by such a field lens depends on the direction in which the eye is looking, but is independent of the aperture of the pupil.
These two types of lens are generally used as alternatives, i.e. in optical correction systems the lenses used belong to one or other of the two above-mentioned types, to the exclusion of lenses of the other type. More rarely, in special cases that are mentioned below, it can happen that both types of lens are combined within a single correction optical system. The present invention relates specifically to such a hybrid system and proposes the use thereof in the following context.
Amongst the various vision disorders that can affect the eye, the present invention relates more particularly to partial or complete loss of accommodation.
Accommodation is the process whereby the eye can focus on objects that are closer or further away, thus enabling objects to be seen in succession and in focus even though they are situated at different distances away, referred to clinically as observation distances or working distances. Conventionally, three observation zones are distinguished: the far vision zone corresponding to observation distances of about 5 meters (m), the intermediate vision zone corresponding to observation distances of about 1 m to about 1.5 m, and the near vision zone corresponding to observation distances of abut 30 centimeters (cm) to 40 cm. Physiologically, the phenomenon of accommodation is based on controlled deformation of the natural lens, leading to a change in the curvature of its surfaces under the effect of the zonule being put under tension or relaxed. When a subject goes from far vision to near vision, the image of the retina goes out of focus and the macular diffusion circle triggers a reflex contraction of the circular portion of the ciliary muscle. This contraction of the ciliary muscle relaxes the tensions of the zonule and allows the lens to become rounder, increasing its sagittal diameter and decreasing its frontal diameter, thereby increasing its spherical power. Conversely, when the eye focuses on a distant point or a point at infinity, the ciliary muscle is relaxed and since the zonule is under tension, the lens flattens.
The accommodation function of the eye can be affected in two main ways.
Firstly in almost systematic manner, during the lifetime of an individual, capacity for visual accommodation decreases, such that most people in their forties or older require different spherical power corrections for near vision and far vision. That is presbyopia. It is recalled at this point that presbyopia is not a kind of ametropia, but is additional to the preexisting ametropia of the patient, if any.
In order to mitigate accommodation deficiency, a person suffering from presbyopia must use one or more types of visual equipment serving not only to correct any ametropia and/or astigmatism, but also to compensate for said accommodation deficiency. Several solutions are available. It is possible to wear either a plurality of pairs of field-lens spectacles, each having a respective single focus, and with the pair of spectacles that is selected being adapted to the observation distance of interest at any given time, or a single pair of field lens spectacles can be used that are multifocal, and preferably progressive, or indeed it is possible to use a pair of contact lenses, i.e. aperture lenses, that are multifocal.
It is known that a multifocal lens possesses a plurality of spherical powers or focuses that differ depending on the zone of the lens that is taken into consideration. Thus, the field lenses of multifocal spectacles have power that varies as a function of the direction in which the eye is looking, with far vision being in the top of the lens and near vision at the bottom. In multifocal contact lenses, the multiplicity of focuses provides a focus depth or caustic that gives the overall visual system of the patient a capacity for pseudo-accommodation. A plurality of images are formed simultaneously on the retina coming from the various far vision, near vision, and optional intermediate vision zones that present different powers. Adaptation with this type of lens is based on the brain discriminating between these multiple images.
When such multifocal lenses have been selected, be they contact lenses or the lenses of spectacles, they must in all circumstances present addition that corresponds to the full requirements or the wearer, since they must mitigate the wearer's lack of accommodation on their own. As a result, if the lack of accommodation is marked, the addition of the accommodation-correcting lens needs to be strong. Unfortunately, investigation work carried out by the Applicant has shown that the addition strength of a lens needs to be considered as a factor contributing to it being rejected by wearers or at least acting as a brake to wearers adapting thereto. This seems to be particularly true for contact lenses.
A patient presenting presbyopia can also envisage wearing contact lenses (i.e. aperture lenses) having a single focus for correcting ametropia in far vision or in near vision, in combination, for the remainder of the observation range, with a pair of spectacles presenting (field) lenses having a single focus. That produces a hybrid system combining both types of lens, field lenses and aperture lenses. Nevertheless, it should be observed that both types of lens then have respective single focuses.
In addition to the physiological disorder of presbyopia, it can happen that the eye suffers from refraction disorders or even from more severe pathologies, leading the ophthalmologist to have recourse to surgery in order to implant an intraocular lens, possibly after removing the patient's natural lens from the eye (where an eye deprived of its natural lens in this way is said to be aphakic). In particular, replacing the natural lens by an intraocular implant constituting an artificial aperture lens has become an operation that is commonplace for cataract surgery and for high degrees of ametropia (in particular of myopia). The drawback is that the aphakic eye loses all natural accommodation power. It is then necessary to give the patient after the operation some kind of accommodation capacity or at least to provide a palliative (the term “compensation” is used). For this purpose, several solutions have been proposed as alternatives.
A first solution consists in implanting a rigid multifocal lens (aperture lens) that does not provide the visual system of the patient with any capacity for accommodation, and to make provision for simultaneously wearing external visual equipment such as a pair of spectacles (field lenses). The surgeon and the patient then need to make a choice between near vision and far vision when determining the power to be given of the intraocular implant. For the remainder of the range of vision, additional correction is essential so the patient must have one or more pairs of single-focus or multiple-focus spectacles. For example, if the patient receives a single focus intraocular implant lens designed for far vision, then that patient does not have the slightest accommodation for intermediate vision or near vision. In order to be able to assess this portion of the visual range, the patient must either wear at least two different pairs of spectacles having single-focus lenses, or a pair of glasses having lenses that are multifocal and preferably progressive. Under such circumstances, which can be considered as being the most practical for the wearer, the multifocal field lenses of the pair of spectacles must present strong addition since they must be capable on their own of mitigating the total loss of the patient's natural accommodation capacity. Unfortunately, studies carried out by the Applicant have led to the conclusion that in combination with the upheaval constituted by the surgery and the artificial implant itself, particularly in patients who are quite old, wearers find it difficult to tolerate high power progressive lenses.
A second solution consists in implanting an intraocular lens presenting accommodation, that is capable of deforming or moving under the action of the ciliary muscle in order to adapt its focus to the visual needs of the patient having the implant, thereby restoring the natural accommodation function previously performed by the natural lens. The results that can be obtained with implants of this type are variable and in any event do not constitute the subject matter of the present invention.
A third solution consists in implanting a multifocal intraocular lens that is supposed on its own to mitigate the patient's loss of accommodation, without it being necessary to have recourse to external visual equipment. The function of multifocal implants is identical to that of multifocal contact lenses, since in both cases they are aperture lenses. The multiplicity of focuses of such an implanted lens provides a depth of focus or caustic that gives the overall visual system of the patient a capacity for pseudo-accommodation. The purpose of implanting such multifocal lenses is naturally to enable the patient to avoid wearing one or more pairs of spectacles, which on the face of it appears to be advantageous. Unfortunately, such multifocal implants are often found to be a failure, either because visual acuity is insufficient, in particular for reading in the near field, or else because the pseudo-accommodation function of such implants does not operate. Under such circumstances, the implant must be replaced or the loss of acuity must be accepted without any accommodation compensation.
From this state of the art, it can be seen that compensating accommodation disorders leads to a persistent problem of intolerance or at least of difficulty in adaptation of subjects to the correcting equipments or systems that are proposed to them.
Above all, the use of multifocal aperture lenses with strong addition seeking on their own to cover the entire range of accommodation leads to the following unwelcome secondary effects:                a long period of adaptation in which the brain learns how to select the sharp image, or possible failure of such adaptation, in particular with the perception of central scotoma, double vision, or image jumping;        halos or night dazzling, associated with the spherical aberration created a portion of the light rays being unfocused; and        loss of luminosity and sensitivity to contrast, associated with light rays being shared between the different focal lengths of the lens.        
Nevertheless, on a daily basis, wearing aperture lenses (intraocular lenses or contact lenses) is a priori the solution that patients prefer because of its comfort and flexibility of implementation.
The use of progressive field lenses is not always satisfactory either, depending on the wearer's disorder and need for addition. The continuous progression of spherical power requires a connection, e.g. on the front face of the lens, between a far vision zone of greater radius of curvature and a near vision zone of smaller radius of curvature, and this inevitably gives rise to zones of reduced optical quality, particularly in the side portions of the lens. The disposition of these zones on the lens defines the field of sharp vision that can be used by the wearer. It is then necessary to find a compromise between field width and peripheral distortion. This is because any enlargement of the sharp field of vision implies an increase in peripheral distortion, which is troublesome in static vision and above all in dynamic vision; and conversely it is possible to limit peripheral distortion only to the detriment of the fields of view that are sharp, which become smaller. It will be understood that this problem is particularly difficult and harmful to adaptation by the wearer when the power added by the lens is considerable.